In plasma etching processes for etching laterally precisely defined recesses into a silicon body, of the type described, for example, in the German Patent No. 42 41 045, the problem of insufficient pocket stability often occurs, i.e., deviations from the desired etching profile arise, in particular at the boundary surface between the etching body and a dielectric interface, for example between the silicon and the silicon dioxide located underneath.
To resolve this, the German Published Patent Application No. 199 57 169 already describes a so-called double-pulse technique, where, via a low-frequency pulsation of a high-frequency modulated carrier signal having a high peak-pulse power at the substrate electrode in the etching chamber of an inductively coupled plasma etching system, this undesired pocket formation is suppressed and, at the same time, a wide process window is achieved for the plasma etching process. In this way, adequate pocket stability is achieved, in particular, at aspect ratios of the etched-in structures of 5:1 to 10:1, as is a certain tolerance to overetching. However, at even higher aspect ratios of the formed trenches and, accordingly, long overetching times, the pocket formation cannot be completely suppressed using this process.
German Published Patent Application No. 199 33 842 proposes also pulsing the inductively coupled plasma source, so that during plasma-discharge pauses, anions occurring at greater rates contribute to the discharging of positive charges of a dielectric etching base in structures having a high aspect ratio. A significant problem arises when pulsing ICP (inductively coupled plasma) plasma sources in that high levels of reflected power occur in the assigned high-frequency generator, since undefined conditions prevail in the plasma during ignition of the plasma discharge. These transient conditions make it difficult to adapt the injected high-frequency power to the plasma impedance. Thus, the igniting of the plasma discharge represents a transition from an electrically capacitively coupled to an inductively coupled mode, which leads to errors in impedance adaptations and, thus, to high levels of reflected power.
To overcome these problems, German Published Patent Application No. 199 27 806 proposes using a feedback circuit of the Meissner oscillator type, including the plasma source as a frequency-determining element, and a high-frequency generator as an amplifier in the feedback path, to release the frequency of the excitation voltage during the transient phases. The drawback of this method is, however, that frequencies can occur outside of the frequency range approved for industrial systems, so that appropriate shielding is required.
Finally, the German Patent Application No. 100 51 831.1 already proposes a device and a method for etching a substrate using an inductively coupled plasma, where a static or time-variable magnetic field is provided between the substrate and the ICP source. The magnetic field is generated by at least two superposed magnetic coils that are traversed in opposite directions by current.